Planodrat tl co



T. G. MARTIN.

AUTOMATIC TELEPHONE SYSTEM.

APPUCATION min MAY 31.1w.

1,315,445. PatentedSept. 9,1919.

P k g i J g t: I {I H "'gu UNITED STATES PATENT OFFICE.

TALBOT G. MARTIN, OF CHICAGO, ILLINOIS, ASSIGNOR TO AUTOMATIC ELECTRICCOMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

AUTOMATIC TELEPHONE SYSTEM.

Specification of Letters Patent.

Patented Sept. 9, 1919.

Application filed May 81, 1917. Serial No. 171,815.

To all whom it may concern:

Be it known that I, TALBOT G. MARTIN, a citizen of the United States ofAmerica, and a resident of Chicago, county of Cook, and State ofIllinois, have invented certain new and useful Improvements in AutomaticTelephone Systems, of which the following is a specification.-

My invention relates in general to automatic telephone systems, but moreparticularly to such systems as comprise a, plurality of exchangesinterconnected by trunk lines; and the object of the invention is toprovide an improved repeater for use in the interoflice trunk circuits,whereby the quality of telephonic transmission for a certain class ofcalls, hitherto below the standard, is improved greatly and is madesubstantially equal to the average quality of transmission in thesystem.

The above indicates in general terms the purpose and scope of theinvention. The manner in which the same is to be used, its mode ofoperation, and the advantages derived from its incorporation in atelephone trunking system of the character described, will all be fullyexplained hereinafter, reference being had to the accompanying drawings.

In the drawings Figure 1 is a schematic diagram illustrating the generallayout of a portion of a system which is well adapted for the use of myimproved repeater, while Fig. 2 is a complete circuit diagram of twoselector switches and a repeater, wired in accordance with theinvention, which are shown only symbolically in Fig. 1.

Referring to the layout Fig. 1, I have shown at the right of the dottedline certain apparatus in one of the main exchanges of the system, whileto the left is shown a branch exchange subsidiary to the main exchange.The main exchange may be a five digit system, and local connections willthen be established through the medium of individual line switches,first, second, and third selector switches, and connector switches. Forexample, the subscriber at substation A may establish connection withthe subscriber at substation A by means of the individual line switch C,the first selector E, the second selector F, the third selector H, andthe connector I. This is the usual and well known percentage trunkingsystem and needs no detailed explanation.

The branch exchange may be a three digit system comprising individualline switches, first selectors and connectors. A calling subscriber atsubstation A may extend a connection to the substation A by means ofline switch C, the first selector E and the connector I. Subscribers inthe branch exchange of course can also call subscribers in the mainexchange, and for this urpose a certain bank level (assuming that t eStrowger type switches are used) of the first selectors, such as theselector E is reserved for trunk lines extending to the main exchange.Thus the selector E not only has access to a plurality of groups ofconnectors, such as the group which includes the connector I, but alsohas access to a group of trunk lines extendin to the main exchange andterminating t ere in first selector switches, termed incoming selectors.'One of these trunk lines is the trunk line 2 which extends by way ofthe repeater R to the main exchange and terminates in the incomingselector E. The incoming selectors have access to second selectors ofthe local trunking system in the main exchange and their banks thereforeare multipled with the banks of certain local first selectors such asthe selector E. For the purpose of giving main exchange subscribersaccess to lines in the branch exchange,acertain bank level of a group ofthird selectors may be assigned to trunk lines extending from the mainto the branch exchange. One of these trunk lines is shown extending fromthe bank of third selector H by way of repeater R to the branch exchangewhere it terminates in the selector E.

The other main exchanges of the system, of which there may be one ormore, are preferably each accessible to subscribers 1n the main exchangeshown from 'a partlcular first selector level. Thus a certain level ofthe selectors E and E may be assigned to a group of trunk linesextending to a second main exchange and terminatmg there n second.selectors. One of these trunk lines 1s the trunk line 3 which extends byway of the repeater R to a second selector (not shown) in the secondmain exchange. It will: be seen now that the main exchange subscriber atsubstation A can extend a connection to the second main exchange by wayof line switch C first selector E, repeater and trunk line 3; while thebranch subscriber at substation A can extend a connection to the samepoint by way of line switch Q, first selector 111 repeater It tru-nkline 2pm.coming selector E, repeater R, and trunk line 3.

From the foregoing 'bri e'tfi description o l a typical trunking systemthe purpose and utility of my invention may easr y be understood. Whenthe subscriber at substation A extends a connection over the trunk line3 his transmitter is supplied with tallking current from the frontbridge or controlling relay of the repeater R'. which must be oisulliciently low resistance to permit the flow of the requisite amountof talking current. But when the branch subscriber at substation Aextends a connection over trunk line 3 transmitter current is suppliedfrom the the front bridge of repeater R", and thus it will be seen thatin this latter case it is extremely desirable that the front bridge inrepeater R- be of high resistance, or there will be excessivetransmission loss. By my invention the repeater R is so designed thatwhen it is seized by a local first selector such as the selector E theusual front bridge will remain across the talking circuit duringconversation for the purpose of supplying transmitter current to thecalling line in the customary manner; but when it is seized by anincoming selector such as selector E, when the called subscriber answersan additional coil of high resistance and impedance is inserted in thebridge, thereby greatly re ducing the transmission loss.

The manner in which this is accomplished can best he explained bydescribing briefly the operation of the incoming selector E and therepeater R during the establishment of a connection, and sucli doscri'tion wi l] now be proceeded with, reference "ein had to Fig. 2. First,however, certs-in pu Bications will be cited as illnstraflin well knownfeatures of the equipment have red, in order to avoid the necessity oftoo elaborate an explanation. The mechanical construction of theswitches is shown in U. S. Letters=Patent Nos. 5%1-5321 and 815,1"7-6',both granted March 13, 1306, to Keith, Ericltsong and Erickson.Cir'cuitsof a modern two wire automatic system are shown in U. 8. PatentNo. 13,901', reissued April 1'3, 1915tto' Nowforth. A repeater halvingthe general functions of the ones heroin shown is disclosed U. 8. Patentto Martin, No 1,136,912, granted A- (11 20 12915:.

To proceed with t! e operation, when the subscriber at mibetatioir Al"removes Ilia m cover to initiate the call, the line switch C is operatedin the usual manner to extend the line of substation A to an idle firstselector switch, which we may assume to be the selector E "he callin rdevice at substation A" is now operate in accordance with the firstdigit of the desired number, resulting in the operation of selector Ewhereby an idle trunk line extending to the main exchange is selectedand connection established therewith. Assn-min that the selected trunkline is the trunk inc 2, when it is seized by the selector E the frontbridge or controlling relay of the repeater It is energized over thecalling subscrihefis loop; and as a result of the energization of thiscontrol'lin relay a bridge is closed across the coiu-iliictors of thetrunk line extendin r between the repeater R and the se lector ill atthe main exchange. The line relay [(1 ol' the selector 1*) isaccordingly energized and upon attracting its armature (loses a circuitfor the slouacting release relay 11. Upon energizing in turn, relay 11prepares circuits for the Vertical and rotary magnets ll and 15 in thewell known manner.

The ceiling subscriber may now operate his calling device in accordancewiththe second digit of the desired number, thereby producing a; seriesof interruptions or inipulses in the circuit of the controlling relay oithe repeater R. These interruptions are repeated in the usual way intothe circuit of the line relay 10 0f the selector E in the main exchange.It follows that the line relay 10' is deenergized a corresponding numberof times. and at each deiinergizati-on' an impulse ilrom ground G istransmitted through the slow acting relay 12 and the vertical magnet 14in series. The vertical magnet in response toithese impulses operates tostep up the switch shaft until the wipers 40; 41 and 4-2 arrive at thehorizontal level of bank contacts which constitute terminals of thetrunk lines extending to the second main exchange.

The relay 1 2 is energized in series with the vertical magnet, and beingslow acting; retains its armature during the series of inn pulses. Uponthe first upward movement of the shaft the off normal springs 1-8 areclosed, thereby eomplbting' an initial energizing circuit tor thestepping relay 13 which may be traced as follows: ground at (2- workingcontact of armature" 2'11 and the said armature, armature 22 and itsworking c intact, ofi' normal s rings 18, andr thewinding of relay 13to' attlery. Upon energizing, relay 13 looks itsel f to ground G at itsupper armature 23 and at its lower armature 9L prepares a circuit forthe rotary magnet 15 which is held openghowever. during the erticalmovement oi the switch by the slow acting relay 12. At this point it;should h s mentioned that by the closure of 011 normal springs 18 theline switching relay 16 and the step ing relay 13 were connected inseries. ut since the junction of the two relays has already beenconnected to ground as described, relay 16 is short circui-ted andremains inoperative for the resent.

When the slow acting re ay 12 denergizes at the end of the series ofimpulses it completes at the back contact of its armature 22 the circuitof the rotar magnet 15 extending as follows: groun at G, working contactof armature 21 and the said armature, armature 22 and its restingcontact, armature 24 and its working contact, and the winding of therotary magnet 15 to battery. Upon energizing, the rotary magnet operatesto rotate the wipers 40, 41 and 42 into engagement with the first set ofbank contacts in the level opposite which they were raised. Near the endof its stroke the rotary magnet opens its interrupter contact, therebybreaking the looking circuit of the steppin relay 13. Upon deenergizing,relay 13 breaks the energizing circuit of the rotary magnet whichaccordingly retracts its armature to again close its interruptercontact. The 0 eration now depends upon whether or not t e first trunkline, the term inals of which are now engaged by the selector wipers, isbusy or not. If this trunk line is busy there will be a round potentialupon the test contact with which the private wiper 41 is in engagement.and this ground potential will serve to maintain the switching relay 16short circuited and to again energize the stepping relay 13. The rotarymagnet circuit is therefore again completed and the foregoing operationsare repeated, whereby the selector wipers are advanced into enga ementwith the next set of contacts. It wil be seen then that the steppingrelay 13 will operate to advance the switch wipers step by step throughthe medium of the rotary magnet as long as the rivate wiper 41 continuesto engage grounded or busy test contacts. When an idle or non groundedtest contact is reached, assumed to be the test contact 44, and therotary magnet retracts its armature, the

switching relay 16 will no longer be short circuited but will instantlyenergize in series with the relay 13. Owing to the high resistance ofrelay 16 relay 13 remains inoperative. Upon energizing, relay 16 opensat its armature 25 a contact in the circuit of the release magnet 27 toguard against the release of the switch upon the deenergization ofrelays 10 and 11; shifts the private wiper 41 from connection with itsupper terminal into connection with its lower terminal at armature 29;and at armatures 28 and 26 disconnects the conductors of the trunk line2 from the windings of the line relay 10 and extends them by wayofwipers 40 and. 42,

bank contacts 43 and 45, conductors 46 and 48, and normally closedcontacts of the reversing relay 106 to the windings of the double woundline relay 100 of the repeater R.

Upon energizing, the line relay 100 closes the usual circuit for theslow acting release relay 101. The relay 101, upon energizing, completesa holding circuit for the selector E as follows: ground at G, Workingcontact of armature 111 and the said armature, winding of relay 105,conductor 47, bank contact 44, private wiper 41, armature 29 and itsworking contact, winding of relay 16, interrupter contact of the rotarymagnet 15 oil normal springs 18, and the winding oi the stepping relay13 to battery. It is to be understood that this circuit is establishedbefore the release relay 11 has had time to deenergize. The relay 105 ofthe repeater which is included in the holding cir" cuit is of lowresistance so that the test contact 44 and its multiples in the banks ofthe other incoming selectors are brought suliiciently near to groundpotential to make the trunk line busy. This ground potential alsoextends over conductor 50 to multiple test contacts in the banks of thelocal selectors such as the selector E where the trunk line 3 is madebusy also. It will be observed that a branch of the above describedholding circuit extends through the coil 17 at the selector E tobattery. This multiple path permits sufficient current to flow throughthe winding of relay 105 at the repeater to energize it, with resultswhich will be described later.

Proceeding now with the operations at the repeater R resulting from theenergizetion of the relay 101, at its armature 112 this relay prepares acircuit for the slow acting relay 102 and at its armature 111 closes acircuit through the polarizing winding of relay 103 as follows: groundat G, working contacts of armature 111, and the lower winding of thesaid relay 103. The lower winding of relay 103 is of high resistance andfew turns, being composed in part of German silver wire, and relay 103is therefore not sufficiently energized to attract its armatures. Astill further result of the energization of relay 101 is the closure ofa bridge across the conductor of the trunk line 3 extending to thesecond main exchange which may be traced as follows: upper conductors oftrunk line 3, armature 110 and its working contact, armature 113 and itsresting contact, upper windin of relay 103, spring 119, contact 120,right liand winding of impedance coil 107, armature 121 of the linerelay 100 and its working contact, to the lower conductor of trunk line3. By the closure of this bridge the line relay of the second selectorat the distant main exchange is energized with'results similar to thosef-Q. 1,ana,m

which were described in the case of the line relay 10 of the selector E.Although the upper wind in of relay 103, which is included in the bridgeas traced, is connected in series with the line relay of the distantsecond selector,- the relay 103- is not operativcly energitacd for itswindings are so connectcd that their energizing effects at this time arein' opposition.

The calling subscriber ma now 0 crate his calling device in accor ancewit the third digit of the desired number, thereby producing anotherseries of interruptions III the circuit of the controlling relay ofrepeater R, which interruptions are now repeated into the circuit of therelay 100 of the repeater R. The line relay 100 is accordinglydei-inergized a number of tii'nes corresponding to the number ofinterrup tions produced in its circuit and at each decnergizationtransmits an impulse to the slow acting relay 102 over the followingpath: ground at G armature 122 and its resting contact, armature 112 andits working contact, and the winding of slow acting relay 102 tobattery. The slow acting relay 102 remains energized continuously duringthe intermittent deenergization of line relay 100 and completes thebridge across the conductors of the trunk line 3 over a new path.exclusive of inductive windings, which may be traced as follows: upperconductor of trunk line 3, armature 110 and its working contact,armature 113 and its working contact, and the armature 121 of the linerelay 100 and its working contact, to the lower conductor of trunk line3. While this new bridge excludes the upper winding of relay 103 and theri ht hand winding of the impedance coil 10 it will be seen that itstill includes the armature 121 of the line relay 100, and it followstherefore that at each denergization of the line relay an interruptionwill be produced in the circuit of the line relay of the second selectorat the distant main exchange. This second selector is accordinglyoperated in the usual manner to establish connection with a trunk lineextending to an idle' third selector. At the end of the series ofinterrnptions the slow acting rela 102 de'encrgizes and the bridgeacross t e trunk line 103 is reestablished over the path first traced.

The calling subscriber may now operate his calling device for theremaining digits in the called number with the result that the selectedthird selector and a connector switch are operated to complete theconnection to the desired called line. After the connection is completedthe called subscriber may be signaled in the customary manner by meansof automatic equipment associated with the connector switch. When' thecalled subscriber answers the signaling operation is stepped nnd a relayin the connector,

which may be the usual answerin bridge, is energized over the calledsubscri ers line to reverse the direction of current flow over theconductors of the trunk line 3. As a result ct this operation thedirection of ourrent flown in the upper winding of relay 103 is reversedand since now the two windings of this relay assist each other it willat once energize: By the attraction of armaturcsi 11-4 and 115 the heavytalking con'duc- 7b tors oi the repeater are extended through totheconductors of the trunk line 3 independent of contacts of relays 100 and102; and by the attraction of armature 116 a circuit is completed in anobvious manner for the reversing relay 106. Relay 106 is accordinglyenergized and at its armature 126 connects the left hand winding ofimpedance coil 107 in series in the bridge across the trunk line 3. Theleft hand winding of impedance coil 107 is of high resistance andimpedance tor the purpose of reducing as far as possible transmissionloss due to this bridge. As a further result of its energiza'tion relay1 06 reverses the conductors 46 and 8 as regards their connections tothe windings of line relay 100, and since the relay 105 is now energizedthese con-ductors after the encrgization of relay 106 will extend to thewindings of the line rela 100 by way of the windings of the inipe ancecoil 104. These circuits may be briefly traced as follows: conductor 16,working contact of armature 127 and the said armaturc. lower winding ofimpedance coil 10-1 and the lower winding of relay 100 to ground.Conductor 418, working contact of armature 128 and the said armature,upper winding of impedance coil 104, and the upper winding of line relay100 to battery. By the inclusion of the impedance coil 104- in the frontbridge of the repeater R the impedance of the bridge is greatlyincreased and a considerable transmission loss which would otherwiseoccur is avoided. The transmission is further benefited by theconnection of condensers 130 and 131 in multiple, respectively, withcondensers 132 and 133. This operation is also brought about by theenergization of relay 106. It will be seen then that when a connectionis extended throu -h the incoming selector E and the repeater that theimpedance and resistance of both the front and back bridges of therepeater are greatly increased with the results already mentioned.Whenthe calling subscriber replaces his receiver the a paratus isrestored to normal in a manner w hich is well understood and which neednot be described herein.

When the subscriber at substation A extends a connection to the secondmain exchange by way of the trunk line 3 the operations are as follows:Upon the removal of the receiver the line switch G operates to extendthe callin line to a first selector switch, which we wi I assume to bethe selector E. The line relay 150 of the selector E is accordinglyenergized over the conductors 152 and 163 to which the calling line hasbeen extended by the operation of the line switch and closes the usualenergizing circuit for the slow acting release relay 151. The relay 151,upon energizing, connects ground to the conductor 144 in order tomaintain the line switch in operated position. By the operation of thecalling subscribers calling device the selector E is now operatedsimilar to the manner in which the selector E was operated to select anidle trunk line extending to the second main exchange, and we willassume in this case also that the trunk line 3 is the one selected. Uponthe seizure of the trunk line 3 the line relay 100 and release relay 101are energized, the latter relay serving to complete a holding circuitfor the selector E which as in the previous case includes the winding ofrelay 105. It will be observed, however, that the selector E has no coilcor responding to the coil 17 of the selector E, and it followstherefore that when the holding circuit for selector E is completed thecurrent flow through the winding of relay 105 will be insufficient toenable it to energize. In other words, relay 105 is a marginal relaywhich will energize only when the repeater R is seized by a selectorswitch which is provided with a coil 17. Needless to say all selectorssimilar to the selector E are provided with such resistance coils whilethe selectors such as the selector E are not so provided.

The apparatus in the distant main exchange is now controlled through themedium of the repeater R in the manner previously described until theconnection is finally established with the line of the calledsubscriber. When the receiver is removed at the called substation therelays 103 and 106 of the repeater are energized as before and with thesame results with one exception, however. Since the relay 105 in thiscase is deencrgized the operation of relay 106 serves merely to reversethe conductors 49 and 51 as regards their connections to the windings ofline relay 100 and does not operate to include the wmdings of theimpedance coil 104 in these connections. This is true because when therelay 105 is deenergized its armatures 117 and 118 short circuit thewindings of relay 104 at their resting contacts. It will be seen thenthat the line relay 100 of the repeater alone is left in bridge of theconductors 49 and 51 during conversation, thereby afi'ording an amplesupply of transmitter current to the calling substation. The reversal ofcurrent flow in the callin line produced by the energization of re ay106 may be used to cause the actuation of a meter or coin collectingdevice in any well known or desired manner.

Having described my invention what I consider to be new and desire tohave protected by Letters Patent will be pointed out in the appendedclaims.

What I claim as my invention is:

1. In a telephone system, two subscribers lines, means including arepeater for setting up a circuit connection from either of said linesto a called line for talking purposes, a controlling relay bridge acrosssaid circuit in said repeater, and means in said repeater effective toincrease the impedance of said bridge during conversation only when aparticular one of said first lines is included in the connection.

2. In a telephone system, two subscribers lines, means including arepeater for setting up a circuit connection from either of said linesto a called line for talking purposes, a controlling relay bridge acrosssaid circuit in said repeater, and means comprising a marginal relay insaid repeater effective to increase the impedance of said bridge durinconversation only when a particular one 0 said first lines is includedin the connection.

3. In a telephone system, two subscribers lines, means including arepeater for setting up a circuit connection from either of said linesto a called line for talking purposes, a controlling relay bridge acrosssaid circuit in said repeater, and means in said repeater effective toincrease the resistance of said bridge during conversation only when aparticular one of said first lines is included in the connection.

4. In a telephone system, a trunk line including a repeater, means forsetting up a circuit connection over said trunk line from two difl'erentoints for talking purposes, a controlling re ay bridge across saidcircuit in said repeater, an impedance coil for insertion in the bridge,and discriminating means in said repeater efl'ective to cause theinclusion of said coil in the said relay bridge only when a connectionis extended over said trunk line from a particular point.

5. In a telephone system, a trunk line including a repeater, means forsetting up a circuit connection over said trunk line from two difierentoints for talking purposes, a controlling re ay bridge across saidcircuit in said repeater, an impedance coil, and diseliminating meanscomprising a marginal relay in said repeater effective to cause theinclusion of said coil in the said relay bridge only when a connectionis extended over said trunk line from a particular point.

6. In a telephone system, two subscribers lines, means includin anelectromagnetic switching mechanism or setting u a circuit connectionfrom either of said ines to a to said switches,

called line for talking urposes, a battery supply bridge across saicircuit included in said mechanism for supplying talking current to acalli line, means or supplymg one of said first ines when calling withtalking current from a different oint, and automatic means forincreasing t e resistance of said bridge when the connection is extendedfrom the calling line last mentioned.

7. In a telephone system, first and second subscribers lines, meansincluding a switching mechanism controllable over either of said lineswhen calling for extending a talking connection to a on led line, abattery supply bridge in said mechanism, and discrlnunating means forincreasing the impedance of said bridge if a particular one of saidfirst lines is included in the connection.

8. In a telc ihone system, first and second subscribers lines, .meansincluding an electromagnetic switching mechanism controlla'ble overeither of said lines when calling for extending a talking connection toa called line, a lntttery supply bridge in said mechanism, means for suplying talkii current by way of said brl go to the saic first line afterthe connection is completed, and means whereby it the said second lineis the calling line the resistance and impedance of mid bfidge isautomatically increased.

9. In a telephone system, first and second subscribers lines, meansincluding an electromagnetic switching mechanism controllable overeither of said lines when calling for extend a talking connection to acalled line, a attery supply bridge in said mechanism, anddiscriminating means for changing the resistance of said bridge if aparticular one of said first lines is included in the connection.

10. In a telephone system, selector switches of two difl'erent kind-s, arepeater accessible to said switches, means including either one of saidswitches said re. ater for connecting calling and called lines, abattery supply bridge in said re, eater, and discrimihating meanswhereby if an established conincludesa selector switch of particularkind the impedance of said bridge is increased.

11. In a telephone ystem, selector switc es of two difl'erent kinds, arepeater accessible to said switches, means including either one of saidswitches and said repeater for connecting calling and called lines, abattery sirpply bridge in said repeater, and discriminoting meanswhereby if an established connection includes a selector switch ofpfll'lilclller kind the resistance of said bridge is altered. V

12 In a telephone system, selector switches of two different kinds, arepeater accessible means including either one of said switches and saidre eater for connecting calling and called llnes, a battery supplybridge in said repeater, discriminating means whereby if an establishedconnec tion includes a selector switch of particular kind the impedanceof said bridge is increased, the said means con'lprising a marginalrelay in said repeater, a conductor including a winding of said relayadapted for seizure by said switches, means in the switches of one kindfor causing a certain current flow in said conductor, and means in theswitches of the other kind for causing a different cur-rent How in saidconductor.

13. In a telephone system, selector switches of two different kinds, arepeater accessible to said switches, means including either one of saidswitches and said repeater for connecting calling and called lines,a'battcry supply bridge in said repeater, discriminating means wherebyif an established connection includes a selector switch of particularkind the resistance of said bridge is altered, the said means comprisinga mar ginal relay in said repeater, a comluclor including a winding ofsaid relay adapted for seizure by said switches, means in the switchesof one kind for causing a certain current flow in said conductor, andmeans in the switches of the other kind for causing a different currentflow in said conductor.

14. In a telephone system, a trunk line consisting of talking conductorsaccessible from two different points, means for extending a connectionfrom either point over said trunk line to a called telephone line, abattery supply bridge connected across the talking conductors of saidtrunk line, and automatic means efl'ective only when a connection isextended over said trunk line from a particular point for increasing theimpedance of said bridge.

15. In a telephone system, a trunk line consisting of talking conductorsaccessible from two difi'erent points, means for extending connectionfrom either point over said trunk line to a called telephone line, abattery supply bridge connected across the talking conductors of saidtrunk line, and automatic means efl'ective only when a connection isextended over said trunk line from a particular point for altering theresistance of said bridge.

16. In a telephone system, two selector switches, a repeater accessibleto said. switches, means including either one of said switches and saidrepeater for connecting calling and called lines in a talking circuit, acontrolling relay bridge in said repeater, whereby if an establishedconnection includes one of said selector switches talking current issupplied to the talking circuit via said last mentioned selector, andmeans whereb if an established connection includes he other of saidselector switches the .im edance of said bridge is increased.

7. In a telephone system, a trunk line ill] including a repeater, meansfor setting up a circuit connection over said trunk line from twodiiierent points for talking purposes, a controlling relay bridge acrosssaid circuit in said repeater, an impedance coil for insertion in thebridge, and a marginal relay in said repeater efl'ective to cause theinclusion of said coil in the said relay bridge only when a connectionis extended over said trunk line from a particular point.

18. In a telephone system, first and second subscribers lines, meansincluding a switchin mechanism controllable over either of said lineswhen calling for extending a talking connection to a called line, abattery supply bridge in said mechanism, and a marginal relay forincreasing the impedance of said bridge if a particular one of saidfirst lines is Included in the connection.

19. In a telephone system, first and second subscribers lines, meansincluding an electromagnetic switching mechanism controllable overeither of said lines when calling for extending a talking connection toa called line, a battery supply bridge in said mechanism, means forsupplying talking current by Way of said bridge to the said first lineafter the connection is completed, and a marginal relay forautomatically increasin the resistance and impedance of said bridge ifthe said second line is the calling line.

20. In a telephone system, first and second subscribers lines, meansincluding an electromagnetic switchin mechanism controllable over eitherof said lines when calling for extending a talkin connection to 3.called line, a battery supp y bridge in said mechanism, and a marginalrelay for changing the resistance of said bridge if a articular one ofsaid first lines is included 111 the connection.

21. In atelephone system, selector switches of two different kinds, arepeater accessible to said switches, means including either one of saidswitches and said repeater for connecting calling and called lines, abattery supply bridge in said repeater, and a marginal relay forincreasing the impedance of said bridge if the established connectionincludes a selector switch of a particular kind.

Signed b me at Chica 0, Cook count State of Illlnois, this 24th a ofMay, 19l

TALBOT MARTIN.

copies of this patent may be obtained for are cents each, by addressingthe "Commissioner of Patents, Washington, D. 0."

